An X-ray CT (Computed Tomography) apparatus is an apparatus which provides the medical information of an object to the user in an image form. This apparatus plays an important role in medical activities such as diagnosis and medical treatment by doctors and the like. When an X-ray CT apparatus starts imaging an object, the X-ray detector detects X-rays emitted from the X-ray tube and transmitted through the object. A DAS (Data Acquisition System) converts a signal output from the X-ray detector into a voltage signal for each channel, amplifies the signal, and converts it into a digital signal, thereby generating pure raw data. Projection data is generated by executing preprocessing for the pure raw data. A storage device stores the projection data together with, for example, supplementary information such as an imaging position and an imaging angle. CT reconstruction processing is executed according to the procedure shown in FIG. 10 in accordance with a display image acquisition request from the user. In CT reconstruction processing, first of all, the user reads out projection data from the storage device in accordance with the image display acquisition request from the user (step S1). The data processing apparatus of the X-ray CT apparatus then executes reconstruction processing for the projection data (step S2). Image processing is executed for the data of the reconstructed image after the reconstruction processing (step S3). Finally, a display device displays the display image data corresponding to the acquisition request from the user (step S4). Conventionally, the procedure for CT reconstruction processing like that shown in FIG. 10 is a fixed procedure, in which intermediate data processed midway through the processing is only temporarily stored in a memory and discarded upon completion of the CT reconstruction processing.
For this reason, even when executing CT reconstruction processing with several data processing conditions, e.g., reconstruction processing conditions, common to CT reconstruction processing executed in the past, the X-ray CT apparatus reads out projection data from the storage device, and executes reconstruction processing and image processing for the projection data. For this reason, even when executing CT reconstruction processing with several data processing conditions common to CT reconstruction processing executed in the past, the X-ray CT apparatus requires almost the same time as that for regular CT reconstruction processing. This makes it difficult to increase the time for diagnosis, medical treatment, and the like.